Fluid flow indicators and meters are widely used for detecting fluid flow in a line and for indicating the rate at which such fluid is flowing. Such indicators and meters are typically installed in series with the flow line and provide a visual or electrical indication of flow per se and/or of the rate of flow in volumetric units per unit of time, e.g., gallons per minute.
Various structures are used to indicate rate of flow and one such "unidirectional" structure is the subject of U.S. Pat. No. 3,805,611 (Hedland). The Hedland flow meter includes a hollow, spring biased, elongated piston with an opening of reduced diameter at one end. A concentric rod extends through the opening and has a tapered portion and a portion of uniform diameter.
The piston coacts with the tapered portion of the rod in a manner such that the area of the annular space between them varies with the relative position of the piston with respect to the rod. That is, as the rate of fluid flow increases, the piston moves in a direction to compress a spring and to cause an increase in the annular area.
Another unidirectional flow meter is the subject of U.S. Pat. No. 4,986,133 (Lake). Among other differences, the Lake flow meter uses a thin disc in place of an elongated piston to coact with a rod having a uniform taper.
Similar unidirectional flow meters use the same fundamental structure of a tapered rod and piston and further incorporate switches and the like for providing electrical signals at particular flow rates. Examples of flow meters of these general types are shown in U.S. Pat. Nos. 3,805,611; 4,349,711; 4,389,901 and 4,487,077.
Exemplary bi-directional flow indicators and transducers are shown in U.S. Pat. Nos. 2,574,866 (Fahrlander); 3,528,288 (Scourtes) and 4,366,718 (Nelson). The Scourtes transducer system uses an armature member which slides along and is guided by a shaft of uniform diameter. Such armature member has a circular knife edge which coacts with the profiled interior surface of the body portion. The diameter of the central part of such surface increases at a uniform rate but such diameter does not change near the outer ends of such interior surface. Each of a pair of springs bears against the armature member for all positions of such member.
The Fahrlander gauge is similar in configuration and operation to the Scourtes system. A difference is that the body opening of the Fahrlander gauge has a generally cylindrical central portion with portions of increasing diameter on either side. Another difference involves the rod-guided compression spring on either side of the rod-guided "travelling" plug of the Fahrlander gauge. The length to which each such spring can extend is limited by flanged bushings which are "stopped" against a shoulder on the support rod. As a result, a single spring bears against the travelling plug whenever such plug moves away from its centered location.
The Nelson transducer has a tube-like core with a central annular ridge. Such ridge moves with respect to a similar ridge on a stationary rod. The Nelson transducer is similar to the Fahrland gauge in that the Nelson transducer also has a compression spring on either side of the moving core. The length to which each such spring can extend is limited by a spring guide bearing against a washer-like spacer mounted on the rod. And like the Fahrlander gauge, a single spring bears against the travelling core whenever such core moves away from its centered location.
While these prior art bi-directional devices have been generally satisfactory for their intended purposes, they are not without disadvantages. For example, the Fahrlander gauge and the Nelson and Scourtes transducers involve a relatively large number of parts. And both the Nelson and Scourtes transducers require electrical "readout" circuits. Such parts and circuits must be made, inventoried, assembled and, perhaps, later serviced. The cost implications are apparent.
Yet another disadvantage is more subtle and involves the way the devices operate at low and high flow rates. The moving component of the Fahrlander, Scourtes and Nelson devices coacts with a curved central profile (the central interior surface of the Fahrlander and Scourtes bodies or the ridge-like exterior surface of the Nelson rod), the cross-sectional area of which changes rather rapidly per unit length. Therefore, for low flow rates and small changes in such flow rates, the incremental change in position of the moving component is slight and more difficult to detect.
On the other hand, the cross-sectional area of the profiles toward the outer ends of such interior surfaces or rod does not change at all. Such profiles are substantially cylindrical. As a result, the moving component may "top out" a high flow rates. It appears the moving component may actually impact an end of the device under high flow.
Another disadvantage of the Fahrlander and Nelson devices is that parts having curved surfaces are usually more difficult to make than parts having surfaces tapered at a uniform rate. This, too, has implications for manufacturing cost.
A bi-directional flow indicator which resolves some of the disadvantages of prior art bi-directional indicators would be an important advance in the art.